This invention relates to flow meter provers, and more particularly to an in-line flow meter prover having increased reliability and accuracy and which may be constructed and operated with a minimum of complexity.
In the use of flow meters to measure the quantity of fluid flowing in a conduit, it frequently is desirable to determine the accuracy of the meter while it is in service, without disrupting the flow of the fluid being measured. A number of devices have been developed and are in use for such purpose, and are known as in-line meter provers.
Provers of the class to which this invention relates operate by causing the fluid stream to pass simultaneously through the meter and through a conduit containing a movable fluid barrier, typically a ball which fits snugly into the conduit or a piston having a similar fit. The barrier device is launched into the fluid stream at an upstream position and travels with the fluid, passing two detection points, and stopping at a downstream position. The barrier is then returned by various means to the upstream position, from which it may be launched again for a following test. The two detection points are normally represented by electrical switches which are actuated by passage of the barrier. A comparison of the volume of the prover in the space between the two detection points with the measurement by the meter of the same volume of fluid serves to determine the meter accuracy.
The type of flow meter to which this class of provers is best applied is one designed to produce a series of electrical impulses, each impulse representing a certain volume of fluid. Flow quantity as measured by the meter is determined by counting the total number of impulses produced as the fluid passes through the meter. Such counting is readily performed by electronic counters which display a number representing the total number of impulses received from the meter.
The purpose of proving is to calibrate the meter so that the quantity of fluid represented by each impulse is precisely known. Additionally, a precise measurement of rate of flow, i.e., volume per unit of time, may be obtained from the prover by dividing the volume between detection points by the time elapsed during the passage of the barrier between the same points. This method of measuring flow rate enables the prover to also be used to calibrate flow meters having an analog output which is a function of flow rate.
With a flow meter designed to produce impulses, proving is performed by connecting the flow meter impulses to a prover counter which is an electronic counter having gating circuits activated by the detector switches in the prover. The gating circuits cut off the incoming pulses until the first detector switch is actuated. The counter then starts and continues counting until the second switch is activated, whereupon the counting is terminated.
The volume of fluid displaced between the two switches is a known volume, having been precisely measured either by a displacement test or by direct measurement of the conduit diameter and the linear distance between the detection points. In the displacement test, which is performed with the prover removed from the flowing stream the volume displaced between the detection points is determined by filling the prover with fluid, causing the barrier to move slowly through the conduit, collecting the displaced fluid in a graduated container, and noting the level in the container at each detection point.
Following a proving test, a numerical factor defining the number of meter impulses per unit of fluid volume is determined by dividing the number of impulses produced during the proving test by the prover volume. This factor is known as a calibration factor, or K factor, and is expressed in terms of impulses per unit volume.
It has been found that the designs employed in the construction of prior art provers may cause measurement errors which limit the accuracy and repeatability achievable with these devices. For example, in prior art provers the displaced volume of the prover will change as the pressure of the fluid causes slight but significant changes in the diameter of the prover conduit. Similarly, conduit dimensions, and hence volume, will change as the temperature of the fluid causes the conduit to expand or contract.
It is commonly accepted practice to calculate a compensation factor to correct for prover volume changes due to the effect of temperature and pressure on the conduit material. For example, in a typical procedure the temperature and pressure of the fluid in the conduit are measured during the proving run. Changes in the dimensions of the conduit relative to a standard temperature and pressure are then calculated, from which a corrected volume is computed.
Several assumptions are made in the above procedure which may result in errors. First, it is assumed that the temperature of the conduit is the same as the temperature of the fluid, which is not the case if there is a difference in temperature between the fluid within the conduit and the ambient air outside the conduit. Further, a standard elastic modulus is assumed for the conduit whereas the modulus actually varies due to variations in the composition of the material from which the conduit is fabricated. Still further, the effects on prover volume due to flanges or other stiffening members at the ends of the conduit are difficult to analyze, and are therefore generally ignored. Finally, the use of compensation calculations is somewhat tedious, presenting opportunities for an operator to make mathematical errors.
Howe U.S. Pat. No. 3,273,375, issued Sept. 20, 1966, discloses calibrating apparatus utilizing a double wall construction in which the measuring conduit is enclosed in an outer housing. An object of this type of construction is to reduce fluid pressure stress on the conduit. Howe shows the measuring conduit supported by a plurality of ribs spaced apart along the length of the measuring conduit and structurally connecting the conduit to the outer housing.
In a conduit that is totally surrounded by fluid, stresses on the conduit imposed by the fluid pressure are virtually eliminated. In the Howe apparatus, however, large pressure differentials exist in the area of the supporting ribs which can cause conduit dimensional distortions. Accordingly, placement of conduit supporting structure adjacent the measuring portion of the conduit contributes to measurement errors as a result of fluid pressure imbalance.
Totally surrounding a conduit with fluid also has the effect of equalizing the temperature of the conduit to the temperature of the fluid. However, in the Howe apparatus described above, the measuring fluid is precluded from contact with the outside surface of the conduit in the area of the supporting ribs. Further, the ribs act to transfer heat from the conduit to the outer housing. These conditions cause temperature gradients which result in conduit dimensional distortions and attendant measurement errors.
In existing provers the launching and return of the fluid barrier device involves many difficult mechanical problems which are not readily overcome. Mechanisms tend to be complex and the prover itself is bulky and costly to construct. Some provers utilize complex reversing valves to reverse the direction of the flow in the conduit and thereby return the barrier to its original position. Other designs utilize devices to retract the barrier and restrain it in the upstream position, as well as to bypass the flow through the piston by means of a poppet valve when the prover is not being used in a proving test. Provers utilizing valves to reverse the direction of the flow are known as bidirectional provers because proving tests may be made with the barrier traveling in either direction. Provers utilizing devices to retract and restrain the barrier are known as unidirectional provers because the fluid and the barrier always travel in the same direction in the conduit.
In the above described designs, there is an interval of time during the operation of either the reversing valve or the poppet valve when a portion of the fluid stream bypasses the barrier. Since the barrier must move with the entire stream during the actual proving of the meter, an additional length of conduit is normally provided upstream of the first detection point to allow the valve to seat properly and to shut off all bypass flow, thus assuring that the barrier is traveling with the full stream flow when it reaches the first detection point.
The Howe invention mentioned above is a bidirectional prover employing a conduit, a free piston as a fluid barrier, and a four-port valve to reverse the direction of fluid flow. Since no means are provided for restraining piston movement during the operation of the valve, a sufficient length of conduit is provided at both the upstream and downstream ends to assure that the valve is closed and that the piston is traveling with the full stream flow before reaching the detection points. From the foregoing discussion it can also be seen that the speed of operation of the valve is a critical factor in the operation of the Howe apparatus. If the valve does not close quickly enough, a portion of the stream will bypass the piston.
Francisco U.S. Pat. No. 3,492,856, issued Feb. 3, 1970, discloses unidirectional flowmeter calibrating apparatus employing a piston within a conduit, where the piston is restrained in the upstream position by means of a complex motor, clutch and cable assembly located upstream of the conduit. A poppet valve, held open by the cable, provides a fluid passage through the piston when the apparatus is not being used for flow measurements. Releasing the cable sets the piston in motion and permits fluid pressure to close the poppet valve. Because the valve operation and the piston restraint are controlled by the same element, namely the cable, there are no means for restraining piston movement during the operation of the valve. Consequently, as in the case of Howe, additional conduit length must be provided to assure that the piston is traveling at full speed before reaching the first detection point. The response time of the poppet valve is also a critical factor in the operation of this apparatus.
Francisco U.S. Pat. No. 4,152,922, issued May 8, 1979, also discloses unidirectional flowmeter calibrating apparatus employing a measuring piston within a measuring cylinder. The piston is restrained in the upstream position by means of a second, retracting, piston mounted within a retracting cylinder located on the upstream side of the measuring cylinder. The retracting piston is connected to the upstream side of the measuring piston. As in the earlier referenced Francisco apparatus, a poppet valve is provided within the measuring piston to form a fluid passage through the measuring piston when the apparatus is not being used for flow measurements. In the upstream position the retracting piston of the latter Francisco apparatus serves the dual function of restraining the measuring piston and holding the poppet valve open. Releasing the retracting piston sets the measuring piston in motion and permits fluid pressure to close the poppet valve. For the reasons stated above, this configuration suffers from the same limitations as the earlier cited references, namely the requirement for additional conduit length, and the critical nature of the poppet valve response time.
Another problem associated with provers which employ movable fluid barriers is caused by the friction developed by the seal between the barrier and the conduit. For example, to reduce the chances of leakage past the movable piston, prior art provers employ seal rings between the piston and the conduit. Friction created by the seals must be overcome to move the piston. As described below the solution to this friction problem is further complicated by the fact that it is desirable to have equal fluid pressure on both the upstream and downstream sides of the piston to avoid affecting the flow rate of the measured fluid. Equal fluid pressure across the piston has the added advantage of preventing leakage around the piston seals.
In those prior art provers employing free pistons, such as the Howe apparatus referenced above, both the upstream and downstream sides of the piston present the same surface area to the fluid pressure. Thus, in this configuration equal fluid pressure on both sides of the piston provides no net force to overcome seal friction. The result is that the free piston will not move until the upstream pressure is greater than the downstream pressure. The magnitude of the friction may readily cause a change in the rate of flow through the prover, introducing an undesirable effect in the total fluid system. This effect of seal friction is especially undesirable where the measured fluid is compressible, because an actual change in fluid volume can occur, causing measurement errors.
The apparatus in the Francisco U.S. Pat. No. 3,492,856 discussed above suffers a similar problem. In fact, this problem is aggravated by the inclusion of a cable assembly attached to the upstream side of the piston. The cable and its drum assembly cause additional drag on the downstream motion of the piston, requiring an even larger pressure imbalance across the piston to overcome both seal friction and cable drag.
In the apparatus disclosed in the Francisco U.S. Pat. No. 4,152,922 discussed above, a rod is connected between the upstream side of the measuring piston and the retracting piston. The area of the rod reduces the effective surface area, and thus the force of the fluid, on the upstream side of the measuring piston, requiring even greater upstream assembly to move the piston. The motion of the measuring piston is further retarded by the friction caused by the seals of the retracting piston. In an attempt to overcome these retarding forces on the measuring piston, the Francisco apparatus must apply pressure to the upstream side of the retracting piston to aid the downstream motion of the measuring piston during a prover test. Further, Francisco teaches that it is desirable to further increase the pressure on the retracting piston beyond that necessary to overcome system drag forces in order to maintain a higher pressure on the downstream side of the measuring piston then on the upstream side thereof. The necessity for this increased pressure is to ensure that the poppet valve employed in the Francisco apparatus remains closed and sealed. Thus the Francisco apparatus requires unequal pressure across the measuring piston for its operation, with the above described undesirable effects. Further, with high fluid pressure in the measuring conduit, the piston rod is subjected to high compression loads which can result in the rod failing as a column.
The Francisco U.S. Pat. No. 4,152,922 also discloses a modification to the apparatus described above in which the retracting rod extends completely through the cylinder from end to end. The object is to reduce the force unbalance created by the area of a rod which is located solely on the upstream side of the measuring piston. Evan with this modification, which requires an additional seal further increasing piston drag force, the Francisco apparatus still requires unbalanced piston pressure to overcome all of the drag forces enumerated above, and to ensure poppet valve closure.
In summary, prior art provers do not operate with equal fluid pressure on both sides of the measuring piston, resulting in flow rate changes and attendant measuring errors.
Means must also be included in fluid barrier type provers for stopping the barrier at the end of its travel. Prior art provers employ mechanical stops which result in an abrupt change in piston motion. For example the Howe apparatus employs rods projecting from the center of both ends of the outer housing to prevent the piston from exiting the conduit. The rods are designed to directly contact the sides of the piston, causing an abrupt stop. The Francisco U.S. Pat. No. 3,492,256 discloses a portion of the poppet valve associated with the piston assembly which is designed to directly contact the downstream end of the cylinder to operate the poppet valve. Apparatus disclosed in both Francisco references employ direct contact between the piston and the upstream end of the cylinder housing to stop the piston motion on the return stroke.
The abrupt changes caused by the stopping mechanisms of the prior art provers tend to produce undesirable perturbations in the fluid system. They also produce high shock loads in the various mechanical components of the system.
As described above, a variety of techniques are employed in prior art unidirectional provers to return the fluid barrier to the upstream position after a prover test. The Francisco apparatus disclosed in U.S. Pat. No. 3,492,856 employs a cable and motor to retract the piston, while in the Francisco apparatus disclosed in U.S. Pat. No. 4,152,922, pressure is applied to the downstream side of the retracting piston, forcing the measuring piston to its upstream position. This latter prover configuration requires means for pressurizing the upstream side of the retracting piston to assist the motion of the measuring piston during a test, as well as means for pressurizing the downstream side of the retracting piston to return the measuring piston to its upstream position after a test. The retracting piston and cylinder arrangement of Francisco is thus complex, requiring high pressure piston seals and a plurality of valves and regulators for proper operation.
Achieving accurate and repeatable measurements with fluid barrier type provers requires that during the proving test there be no fluid leakage past the barrier seals or past any bypass valve mechanisms. In U.S. Pat. No. 4,152,922, Francisco discloses methods of testing for seal leakage which require either shutting off fluid flow to the entire system, or removing the prover from the system and performing a separate bench test. In either case the flow of the fluid being measured is disrupted and consequently these leak test methods cannot be performed while the prover is in operation. Thus, there is no way of assuring the integrity of the seals during an actual proving test.
It is accordingly an object of the present invention to provide a new and improved flow meter prover which requires no external conpensation for pressure or temperature changes;
It is yet another object of the present invention to provide a new and improved flow meter prover which may be operated with equal fluid pressure on both sides of the fluid barrier;
It is yet another object of the present invention to provide a new and improved flow meter prover in which the integrity of the seals affecting prover accuracy are continuously monitored while the prover is functioning.